1. Field of the Invention
The present invention relates to a process for the acetoxylation of olefins, in which a gaseous reaction stream containing an olefin, acetic acid and molecular oxygen is passed over at least two catalyst zones of differing reactivity arranged in series. Furthermore, the invention relates to a catalyst system for the gas-phase oxidation of olefins to form acetoxylated products, in particular for the preparation of vinyl acetate, which comprises at least two catalyst zones which are arranged in layers and have a reactivity matched to the course of the reaction.
2. Description of the Related Art
Processes for the acetoxylation of ethylene in the gas phase are of particular industrial interest. It is known from the literature that acetoxylations can be carried out industrially by catalytic gas-phase oxidation of olefins such as ethene or propene in fixed-bed reactors. These reactions are preferably carried out in shell-and-tube reactors. Unsaturated esters such as vinyl acetate, inter alia, are prepared by means of this reaction. In general, this reaction is carried out by passing a gaseous mixture comprising molecular oxygen, an olefin and acetic acid through a reactor. Use is usually made of a shell-and-tube reactor in which a plurality of reaction tubes are arranged in parallel and in each of which a uniform catalyst charge is located. The excess heat of reaction involved is removed by means of a heat transfer medium. An example of such a reactor is a boiling water reactor.
As catalytically active components of the catalysts used here, it is possible to use, inter alia, palladium and/or compounds thereof and alkali metal compounds and also additionally gold and/or compounds thereof (Pd/alkali metal/Au system). Systems composed of cadmium and/or compounds thereof (Pd/alkali metal/Cd system) or barium and/or compounds thereof (Pd/alkali metal/Ba system) and also systems containing palladium, alkali metal compounds and mixtures of gold and/or cadmium and/or barium are also used. All these systems are usually present on a suitable support material. Industrially, preference is given to using palladium/gold catalyst systems.
As alkali metal compounds, use is usually made of potassium compounds, for example potassium acetate. During operation, introduction of the alkali metal compound is usually carried out in order to compensate corresponding alkali metal losses within the catalyst bed.
Despite various methods of regulating the reaction temperature known from the prior art, local formation of a stationary temperature peak in the catalyst bed in which a higher temperature prevails than in the remainder of the catalyst bed can occur.
These temperature peaks (known as “hot spots”) bring about a series of undesirable effects during the course of the reaction. Firstly, they limit a further increase in the starting material concentration (loading), which is equivalent to limitation of the space-time yield, and secondly an increase in total oxidation in the reaction mixture can occur (reduction in selectivity to the target product). The latter is, in particular, reflected in a higher specific raw material usage and has a significantly negative effect on the economics of the process. Furthermore, hot spots bring about premature aging of the catalyst.
Particularly in the case of fresh catalyst beds and also in the case of reactions with a high oxygen loading, irreversible damage to the catalyst can occur in this sensitive range when heat removal is insufficient.
Furthermore, a decrease in the starting material concentration combined with a simultaneous increase in the product concentration occurs as the reaction progresses along the catalyst bed. The increasing product concentration can then again lead to inhibition of product formation and thus to a decrease in selectivity and conversion.
Processes for the acetoxylation of ethylene in the gas phase which lead high yields of vinyl acetate are of great economic importance.
WO 2008/071610 discloses a process and a catalyst system comprising a catalyst which comprises palladium, gold and potassium acetate and is applied to an SiO2 support having a large surface area and can be operated at a space-time yield of more than 800 [g (VAM)/l cat*h] at ethene selectivities of greater than 92% and at a low degree of formation of ethyl acetate relative to vinyl acetate.
U.S. Pat. No. 5,179,056 discloses a process for preparing vinyl acetate by reaction of ethylene and acetic acid in the presence of an oxygen-containing gas over a highly reactive palladium/gold coated catalyst.
U.S. Pat. No. 6,399,813 discloses a highly active fluidized-bed vinyl acetate catalyst on a support composed of inert microspheroidal particals composed of silicon oxide, zirconium oxide or aluminum oxide and having a defined pore distribution.
In view of the great economic importance of acetoxylated products and the high-performance catalysts known from the prior art, there is a great need to optimize the course of the reaction in respect of conversion, selectivity and life of the catalyst.
WO 2007/101749 and WO 2006/042659 disclose, for example, synthesis reactors for preparing vinyl acetate monomer with increased selectivity and productivity, in which gaseous ethylene and acetic acid and also an oxygen-containing gas react catalytically, with the synthesis reactors being a wall reactor and the catalytic synthesis being carried out in a plurality of reaction spaces and at least one wall of the reaction spaces being coated with catalyst and at least one wall of the reaction spaces being indirectly cooled.
Reactions which describe a series arrangement of at least two reactors which can be charged with catalysts of differing reactivity are likewise known from the prior art. However, a disadvantage of this arrangement is the large outlay in terms of equipment.
It was an object of the invention to provide a novel process for the acetoxylation of acetic acid using a catalyst system by means of which, in particular, high space-time yields, high selectivities with low by-product formation and an ideally isothermal temperature profile in the catalyst bed are achieved, as a result of which a lengthening of the life and also short start-up times of fresh catalyst can be achieved and at the same time the use of conventional shell-and-tube reactors can be retained.